1. Field of the Invention
This invention relates generally to the power supply systems that include DC-to-DC conversion operations. More particularly, this invention relates to an improved circuit design and configuration to achieve better power conversion efficiency, broader operation ranges and improved soft switch performance.
2. Description of the Prior Art
Conventional art of design and manufacture of the resonant DC-to-DC converter are confronted with the limitations of low power conversion efficiency and narrow operation ranges. Specifically, in a conventional pulse-width modulated (PWM) converter, for the purpose of achieving a hold-up time under the circumstance of a drop of input voltage, the duty cycle and other operational parameters must be compromised to achieve the hold-up requirement under a low input voltage condition. The power conversion efficiency is sacrificed for normal operation with input voltage within its normal range. This difficulty of not able to optimize the circuit design for properly selecting the parameters of resonant network most suitable to a normal input voltage range leads to wastes of power. Additionally, as will be further discussed below, conventional resonant converter has limited ranges of input and output voltages and that often limit the application flexibility of a DC-to-DC converter when implemented with a resonant circuit.
There are two types of resonant converters, namely the series resonant converter and parallel resonant converter. Implemented with a full-bridge or a half-bridge structure, an inductance-capacitance (LC) resonant tank was used to create conditions for lossless turn-on or turn-off of the semiconductor switches. FIG. 1 shows a series resonant converter implemented with a half-bridge structure with the load connected in series with the resonant tank. In contrast, FIG. 2 shows a parallel resonator converter implemented with a half-bridge structure with the load arranged in parallel with the resonant capacitor. Generally, when switching frequency is above the resonant frequency, the switches turn on at zero voltage condition, thus eliminating the turn-on switching losses. In order to regulate the output voltage, the series resonant converter and parallel resonant converter apply a variable switching frequency control method. For a series resonant converter, the major disadvantage is that it requires a relatively large frequency range to regulate the output for a wide load range and the output could not be well regulated under no-load condition. With a parallel connection of the resonant tank and the load, a parallel resonant converter can regulate the output voltage under no-load condition. However, the circulation energy is significantly high. As a result, the power conversion efficiency decreases rapidly as the load is reduced. Also, the performance of the series resonant converter and parallel resonant converter are both limited by the relatively narrow ranges of the input voltage.
FIG. 3 shows the circuit of LCC resonant converter. LCC resonant converter derived from parallel resonant converter by adding a series resonant capacitor Cs. Compared to parallel resonant converter, the circulation energy of LCC resonant converter is reduced and the performance of voltage regulation is improved. However, the LCC resonant converter is still limited by the relatively narrow range of input voltages.
Therefore, an improved resonant converter for broadening the range of the input voltages and to improve the conversion efficiency is required to resolve these difficulties. Specifically, a new circuit architecture is required that would preserve the soft switching characteristics while allow the circuit design optimization based on the normal operation conditions without being limited by the holdup requirement during a drop of the input voltage.
It is therefore an object of the present invention to provide a novel configuration and method of design and manufacturing of a DC-to-DC converter for improving the conversion efficiency while preserving the soft switching characteristic and allowing the circuit design to be optimized for a normal operation condition. The new and improved DC-to DC converter can therefore enables those of ordinary skill in the art to overcome the difficulties of the prior art.
Specifically, it is an object of the present invention to provide a configuration and method by providing a LLC resonant network to a DC-to-DC converter to have dual characteristic resonant frequencies such that the output voltage can be controlled by adjusting the switching period of a pair of input switches. The range of input and output voltages can be more flexibly adjusted based on these operational and control characteristics and the circuit design can be conveniently optimized based on a normal operation condition.
Briefly, in a preferred embodiment, the present invention discloses a DC-to-DC converter. The DC-to-DC converter includes a square wave generator for generating a sequence of output voltages having a waveform of square wave. The DC-to-DC converter further includes a resonant tank connected to the square wave generator comprising a series capacitor connected to a series inductor and a parallel inductor. The DC-to-DC converter further includes a transformer having a primary side connected in series with the series inductor and connected in parallel to the parallel inductor. The transformer further includes a secondary side for connecting to a rectifying circuit for providing a rectified DC voltage to an output load circuit. The series capacitor functioning with the series inductor to provide a first characteristic resonant frequency represented by ƒs, and the series capacitor functioning with the series inductor and the parallel inductor to provide a second characteristic resonant frequency represented byƒm wherein ƒs greater than ƒm. In a preferred embodiment, the first characteristic resonant frequency is ƒs=1/(2xcfx80{square root over (Ls+L Cs+L )}) and the second characteristic resonant frequency is ƒm=1/(2xcfx80{square root over ((Ls+L +Lm+L )Cs+L )}) wherein Cs representing a capacitance of the series capacitor, Ls representing an inductance of the series inductor and Lm representing an inductance of the parallel inductor.
These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment that is illustrated in the various drawing figures.